The present invention is a method to determine if a crude-oil pipestill is operating optimally for the particular crude-oil feedstream that is being fed to the pipestill.
All crude oils have varying quantities of material in their boiling range fractions, and each fraction will have different physical properties that are determined by the specific molecular species present. The combination of these two factors, volume and physical properties, determine the overall quality of a crude and is a significant factor in determining the value for the material. The crude quality is also used to define the operational settings for a refinery as that crude oil is processed.
In the petrochemical industry, crude quality had traditionally been assessed using a crude assay. When a crude oil is assayed, it is distilled in two steps. A method such as ASTM D2892 (see Annual Book of ASTM Standards, Volumes 5.01-5.03, American Society for Testing and Materials, Philadelphia, Pa.) is used to isolate distillate cuts boiling below approximately 650° F. (343° C.). The residue from this distillation is further distilled using a method such as ASTM D5236 to produce distillate cuts covering the range from 650° F. to approximately 1000-1054° F. (343° C. to 538-568° C.) and a vacuum residue cut. At a minimum, cuts corresponding to typical products or unit feeds are typically isolated, including LPG (Initial Boiling Point to 68° F.), LSR (68-155° F.), naphtha (155-350° F.), kerosene (350-500° F.), diesel (500-650° F.), vacuum gas oil (650° F. to 1000-1054° F.), and vacuum residue (1000-1054° F.+). Each distillate cut is then analyzed for elemental, molecular, physical and/or performance properties. The specific analyses conducted depend on the typical disposition of the cut. The data derived from these analyses will typically be stored in an electronic database where it can be mathematically manipulated to estimate crude qualities for any desired distillation range. Commercial crude assay libraries are available from Haverly Systems Inc., and HPI Consultants Inc., both of which provide tools for manipulating the data, as does Aspentech Inc. Assay data is published by Crude Quality Inc., by Shell Oil Company, and by Statoil. The property versus distillation temperature data is typically fit to smooth curves that can then be used to estimate the property for any desired distillation cut.
A detailed crude assay can take several weeks to months to complete. As a result, the assay data used for making business decisions, and for planning, controlling and optimizing operations is typically not from the cargoes currently being bought, sold or processed, but rather historical data. The assays do not account for variations between cargoes that can have a significant effect on operations. K. G. Waguespack (Hydrocarbon Processing, 77 (9), 1998 Feature Article) discusses the sources of oil quality variation, their effect on refinery operations, and the need for improved analytical technology for use in crude oil quality monitoring. Wagusepack lists sources of crude oil variability, both over time and during its transport life as: aging production reservoirs; changes in relative field production rates; mixing of crude in the gathering system; pipeline degradation vis-à-vis batch interfaces; contamination; and injection of significantly different quality streams into common specification crude streams. Such variations can cause significant changes in the value of the crude oil, and in the products that can be made from it.
Refinery Crude Units, also referred to as Pipestills, separate crude oils into their constituent boiling range fractions at different boiling point temperatures (cut points) that then become feeds to other refinery process units or for blending into finished petroleum products. The respective cut points are determined by economic factors as well as the quantity of material anticipated to be available in each of the boiling range fractions. Refinery operation is optimized to maximize recovery of the highest valued streams and products as determined by sophisticated mathematical models of the plant operation using the most recent crude assay.
Deviations from the optimum operation can be costly and units are constantly monitored to keep them within the operating targets. As deviations are observed, plant personnel attempt to understand the underlying causes so that they may be corrected. There are many possible causes for these deviations. These may include mechanical problems, such as fouling of distillation tower internals and/or associated heat exchanger equipment, mechanical damage to tower internals, and faulty instrumentation. The deviation can also be caused by incorrect control settings. Identifying the root cause for the deviation may be a difficult and time-consuming task. Complicating the analysis is that while optimum operation is determined using a laboratory assay, the delivered crude qualities can deviate, sometimes significantly, from those specified in the assay. In addition, feed streams are often a blend of different crudes and the precise percentage of each crude in the blend may not be known with a high degree of accuracy. Plant personnel must decide whether the deviation is due to sub-optimal plant operation or is the result of the normal variation in crude quality and/or make up of the crude blend. This uncertainty can result in delays or inaction towards rectifying underlying operational problems resulting in continued sub-optimal operation. The ability to confirm or eliminate crude quality as an underlying cause for the observed deviation can therefore accelerate problem resolution.